A field-effect transistor (FET) is a transistor, which controls electric current between a source electrode and a drain electrode by providing a gate for a flow of electrons or holes with a channel of an electric field.
The FET has been used as a switching element, or an amplifying element, because of properties thereof. As the FET has a flat structure as well as using low gate electric current, the FET can be easily produced and integrated compared to a bipolar transistor. For these reasons, the FET is an essential part used in integrated circuit of current electric devices. The FET is applied in an active matrix display as a thin film transistor (TFT).
As a flat panel display (FPD), a liquid crystal display (LCD), an organic electroluminescent (EL) display, and electronic paper have recently been in use.
These FPDs are driven by a driving circuit containing TFT using amorphous silicon or polycrystalline silicon in an active layer. There are demands for increasing the size, improving the definition and image quality, and increasing driving speed of the FPD. To this end, there is a need of a TFT, which has high carrier mobility, a high on/off ratio, small variations in the properties thereof over time, and small variations between the elements.
However, TFTs using amorphous silicon or polycrystalline silicon for an active layer have advantages and disadvantages. It is therefore difficult to achieve all of the requirements at the same time. To respond to all of the requirements, developments of TFTs using an oxide semiconductor in an active layer, the mobility of which is expected to be higher than amorphous silicon, have been actively conducted. For example, disclosed is a TFT using InGaZnO4 in a semiconductor layer (see, for example, NPL 1).
The TFT is required to have small variations in threshold voltage.
One of the factors to vary the threshold voltage of the TFT is absorption and desorption of moisture, hydrogen, or oxygen contained in the atmosphere to or from a semiconductor layer. Therefore, a passivation layer is provided to protect the semiconductor from moisture, hydrogen, or oxygen contained in the atmosphere.
Moreover, the threshold voltage is also changed by repeating on, and off of the TFT for numerous of times over a long period. As for a method for evaluating variations of threshold voltage as a result of driving for long period and numerous of time, a bias temperature stress (BTS) test has been widely carried out. This test is a method, in which voltage is applied between a gate electrode and source electrode of a field-effect transistor for a certain period, and variations in the threshold voltage during this period is evaluated, or a method, in which voltage is applied between a gate electrode and source electrode, and between a drain electrode and source electrode for a certain period, and variations in the threshold voltage during this period is evaluated.
Several passivation layers are disclosed in order to prevent variations in the threshold voltage of the TFT. For example, disclosed is a field-effect transistor using SiO2, Si3N4, or SiON as a passivation layer (see, for example, PTL 1). It is reported that the field-effect transistor, in which this passivation layer is stably operated, without transistor characteristic of which is influenced by an atmosphere, such as vacuum, and the atmospheric air. However, the data associated with the BTS test is not disclosed therein.
Moreover, disclosed is a field-effect transistor using Al2O3, AlN, or AlON as a passivation layer (see, for example, PTL 2). It is reported that the field-effect transistor can suppress variations in transistor characteristics thereof by preventing incorporation of a semiconductor layer with impurities, such as moisture, and oxygen. However, the data associated with the BTS test is not disclosed therein.
Moreover, disclosed is a field-effect transistor using a laminate film containing Al2O3 and SiO2 as a passivation layer (see, for example, PTL 3). It is reported that the field-effect transistor using this laminate film as a passivation layer can prevent incorporation and absorption of moisture into the semiconductor layer, and the transistor characteristics thereof do not change after a storing test in a high temperature-high humidity environment. However, the data associated with the BTS test is not disclosed therein.
Moreover, disclosed is a field-effect transistor using a single layer film of SiO2, Ta2O5, TiO2, HfO2, ZrO2, Y2O3, or Al2O3, or a laminate film thereof as a passivation layer (see, for example, PTL 4). It is reported that the field-effect transistor, in which the aforementioned passivation layer is formed, can be prevent desorption of oxygen from an oxide semiconductor, and can improve reliability. However, the data associated with the BTS test is not disclosed therein.
Furthermore, disclosed is a field-effect transistor using Al2O3 as a passivation layer (see, for example, NPL 2). The result of the reliability evaluation of the field-effect transistor, in which the aforementioned passivation layer is formed, performed by the BTS test is reported, but the variation value of the threshold voltage (ΔVth) is large relative to the stress time elapsed. It is therefore cannot be said that the reliability of the field-effect transistor is sufficiently secured.
In any of the aforementioned techniques, the reliability evaluation by the BTS test is not sufficient.
Accordingly, there is currently a need for a field-effect transistor, which has a small variation in a threshold voltage at the BTS test, and exhibits high reliability.